1. Field of the Invention
This invention is directed to coatings for implantable medical devices, such as drug eluting vascular stents.
2. Description of the State of the Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress the atherosclerotic plaque of the lesion to remodel the lumen wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings, which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining, and to re-duce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain vascular patency.
Stents are used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the passageway. Typically, stents are capable of being compressed, so that they can be inserted through small vessels using catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents that have been applied in PTCA procedures include stents illustrated in U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
Pharmacological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an effective concentration at the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred treatment method in that smaller total medication levels are administered compared to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves better results. One proposed method for medicating stents involves using a polymeric carrier coated onto the stent's surface. A solution, which includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent. The solvent is allowed to evaporate, leaving a coating of the polymer and the therapeutic substance impregnated in the polymer on the stent surface.
One polymer that can be used for making stent coatings is poly(n-butyl methacrylate) (PBMA). This polymer is durable, and biologically compatible, and can be used either as a polymeric matrix carrying the drug, or as a topcoat membrane regulating the release rate of the drug. However, PBMA has properties that can be improved. In particular, the release rate of some hydrophobic, medium-molecular-weight drugs, such as EVEROLIMUS, from the PBMA-based stent coatings can be too low. As a result, a higher drug-to-polymer ratio may be required, which is undesirable. At high drug-to-polymer ratios, some of the PBMA-based drug and polymer compositions can have insufficient elongation at high strains that expansion of the stent creates in the coating.
The embodiments of the invention provide stent coatings that are free of the above-described deficiencies and possess other beneficial properties.